Some ebook readers are unable to show certain ebook file formats. To make sure that you can read the book or article you wanted to, you can convert them to MOBI. That way you can read EPUB, AZW, LRF, FB2 or even Microsoft Word and ODT files.
MOBI is one of the most common ebook file formats. It's open-source and thus used by many different ebook readers. Thus, you can often read any kind of book when you convert EPUB to MOBI or AZW to MOBI. Of course, you can convert your own writing as well. Just convert your DOC to MOBI or turn any PDF into an ebook file.